Throughout this application, various references are cited in square brackets to describe more fully the state of the art to which this invention pertains. A list of these references is provided after the Examples. The disclosure of these references is hereby incorporated by reference into the present disclosure.
Lignin is the most abundant renewable resource next to cellulose and it is massively generated from the papermaking and emerging cellulosic ethanol industries. It is anticipated that upcoming ethanol biorefineries will generate large quantities of lignin (about 225 million tons in North America) with better chemical properties compared to paper pulp lignin (75 million tons) [3]. Lignin is a light weight (half the density of talc and calcium carbonate), stiff and brittle biopolymer, very little used as thermoplastic filler [1, 2].
About 2% of the lignin generated from the paper and biorefinery industries is used in value added applications, such as the isolation of chemicals, while the rest is used as low grade burning fuel [3]. Lignin also finds some application in adhesives and asphalts [4], chemicals synthesis, as phenol replacement in phenol-formaldehyde formulations, and in polyurethanes [3]. However, very limited studies have been made on the application of lignin in polymer composites or polymer blends. Lignin-polymer blends, lignin based polyurethane, lignin based epoxy composites, lignin-rubber composites and lignin thermoplastic composites have been reviewed by Kumar et al. [3]. Lignin has been used in the thermoplastic blends of polyethylene terephthalate (PET, polyethylene oxide (PEO) polypropylene (PP), Polyethylenes, polyvinyl alcohol (PVA), polystyrenes (PS), and Polyvinyl chloride (PVC). Lignin acts as a coupling agent depending upon the functionality of polymers [5]. Lignin is also used as a compatibilizer in jute fabric-PP composites [6]. Lignin incorporation slightly improved the PP-jute adhesion. It has been reported that lignin acts as a beta nucleating agent, fire retardant and toughening agent for neat PP. For a better economy and environmental sustainability, disposal and value added application of lignin should be considered critically.
A very limited study has been done on lignin based biodegradable polymer composites. Baumberger et al. [7] studied 20% lignin filled starch composites. Lignin, wood flour based polycaprolactone (PCL) composites has also been reported [8]. Maleic anhydride grafted PCL was used as a compatibilizer for improving tensile properties. However, nothing was mentioned on impact performance of composites. Lignin played a key role as nucleating agent in improving the thermal properties of lignin-polyhydroxy butyrate (PHB) composites [9, 2]. Lignin is also used as adhesion promoter in cotton fiber reinforced polylactic acid (PLA) composites [10]. Thermal and mechanical properties of lignin/Poly(L-lactic acid) (PLLA) blend have been studied by Li et al. [11]. Li, et al. reported a decrease in the tensile strength and elongation of blends with lignin incorporation. Li et al. reported that tensile modulus remained almost constant up to 20% lignin incorporation. Lignin accelerated thermal degradation when lignin content reached 20%. PLA and polyhydroxyalkanoates (PHAs) are the widely used biopolymers but they are facing challenges due to their inferior impact performance.
Blending of polymers [21-23] and/or hybridization of fillers [24-26] are interesting material science to improve properties of composites by balancing strength, stiffness and toughness. So, a binary or ternary blend of lignin with polymers with or without fiber reinforcement could be very promising in material applications.
There is still a need in the art to develop a low cost biodegradable material from polymer and lignin (primarily an industrial waste produced in large scale). The present invention meets the aforementioned needs by providing for a low cost bioplastic material comprising a polymer and lignin with improved mechanical and thermo-mechanical properties.